1. Field of the Invention
The invention relates to process equipment employed for working brittle materials (semiconductors, glass, ceramics and others) with the aid of an abrasive material, and more particularly, to apparatus for preparing and feeding abrasive-containing suspensions into the zone of action of work tools of polishing and finishing lathes.
The invention can be used for polishing semiconductor slices.
2. Description of the Prior Art
Basic conditions for ensuring a uniform working of a surface through a polishing operation by means of a non-bound abrasive are such that a constant solid (abrasive materials particles)-to-liquid phase ratio in a suspension fed into the zone of action of polishing disks, a constant pH (acidity) of a solution and a constant consumption of the suspension in the zone of working must be maintained.
Deviations from a specified ratio between the solid and the liquid phases in the suspension lead, for example, in the event of an increase in the number of abrasive particles, to scratches on a surface being polished. A lesser suspension flow rate (lesser consumption) results in the impaired quality of the surface being polished, with scratches, and irregularities due to appear thereon.
There is known an apparatus for preparing and feeding by gravity an abrasive-containing suspension into the zone of action of work tools of polishing and finishing lathes.
This apparatus, relatively simple in design, comprises a storage reservoir for the suspension and a distributing unit. The rate of flow of the suspension in this apparatus is governed by the amount thereof contained in the storage reservoir, and to ensure a constant flow rate, it is necessary to maintain this amount constant throughout the working cycle, thus, if no automation means are available, it requires additional attending personnel.
There is also known an apparatus for preparing and feeding an abrasive-containing suspension into the zone of action of work tools of polishing and finishing lathes, wherein the flow rate of the suspension is maintained constant by means of pumps and valves provided in a suspension feeding mains (cf. U.S. Pat. No. 4,059,929, dated 1977). The known apparatus comprises a unit for preparing the suspension from starting components, consisting of two tanks adapted to contain a suspension concentrate and a solvent fed separately thereinto. Each of the tanks is connected to its distributing unit formed with a pipe of constant cross section and branch pipes for feeding the suspension through mixing heads to each work tool. Each distributing unit communicates with its individual tank by means of outlets. The apparatus also comprises a waste suspension collecting tank communicating with a unit for cleaning the waste suspension where the solid abrasive particles are settled to the bottom while the separated solvent is pumped into a corresponding tank of the unit for preparing the suspension for re-use. This system operates in the manner below.
The suspension concentrate and the solvent from respective tanks are fed by pumps to the distributing units having branch pipes for feeding the starting components into the mixing heads, where the suspension concentrate is mixed with the solvent.
The resultant suspension from the mixing heads is fed to the working zone. The solid-to-liquid phase ratio can be adjusted by an appropriate variation of the delivery of the pumps. The excess components from the distributing unit are returned to the starting tanks.
The above apparatus for preparing and feeding the suspension is capable of providing a stable supply of the suspension to one lathe only.
When two or more lathes are to be served, it becomes rather difficult to provide for the same solid-to-liquid ratio in a suspension fed to all lathes.
This can be explained by the fact that, as the suspension flows along the pipe of the distributing unit, a portion thereof is passed through branch pipes to the individual lathes, so that the flow rate of the suspension drops to a minimum at the last lathe. The risk is thus increased of the abrasive particles settling in the pipe near the last lathes and of the liquid phase of the suspension segregating to individual fractions by specific gravity.
These phenomena bring about a variation in the solid-to-liquid ratio in the suspension which is difficult to control. The probability of settling of the solid particles is particularly great at points of connection of the branch pipes having a smaller cross section than the pipe of the distributing unit. Vortical fluxes which tend to appear at these points of connection additionally slow down the flow of the suspension, this resulting in an uneven dispersion of the solid abrasive particles, accumulation of mud and choking or plugging up of the branch pipes.